The proof is in the pudding. Don't count your chickens before they're hatched. A bird in the hand beats two in the bush.
There are so many old adages that say virtually the same thing. Yet, there is no shortage of persons who come to my office--usually women--who are angry and puzzled because a man with whom they've been involved didn't do as he said.
The oldest rule of psychology is that past behavior is the best predictor of future behavior. If you want to know what someone's future behavior is likely to be, don't listen to what the person promises. Look at his or her past behavior. Yes, people can change. But they usually don't. Trust me; if everyone who could change their behavior came to psychotherapy in order to change that behavior, I would have a lengthy waiting list and so would every other therapist, and we would all be rich. I fly economy class. Most people with negative behaviors and personality problems never go to therapy and aren't interested. A man who is a chronic cheat or a habitual liar is unlikely to become a different person. In most cases, his cheating and lying works for him and he doesn't care how it affects others.
You (probably) deserve better. This advice is free.